Hi. First time posting here.
Wondering if this is possible:
I have a UML1000 light. It came with a single protected 18650 3.7v 2600mAh –3100mAh, rechargeable battery.
Wondering if I could use that battery for a light that uses 2x CR123 batteries.
Or could I use 2x CR123 batteries in the light that uses the 18650 battery?
How do you fit a 18650 in that?
http://www.amazon.co.uk/UML1000-Urinal-Bottle-Secure-Measuring/dp/B00MBES4Z6
Depends on the Driver.
2 CR123 have 6V, one 18650 has only 3.x-4.2V
I wouldn’t try that if you are not able to change the driver if it gets damaged.
Or if you are absolutely sure that the driver can handle both.
If you fit a 18650 in a 2*CR123 light, it shouldn’t be any damage, but the other way around is dangerous.
So this is the light I’m using
UML1000
Same light as the HOG Morph 1000 Backup.
So… I’m kind of new with all this. Not sure what a “driver” is.
So you’re saying 2x CR123 batteries would be too much for a light that is made for the 18650, but the 18650 should be ok in a light that uses the 2x CR123 batteries, but would probably be underpowered since it isn’t as much voltage?
Thanks for the quick reply… and humor.
They don't seem to listing any voltage range for that light. I probably wouldn't risk the 2 x CR123 unless you can get email confirmation from them that it is OK.
Many of these budget lights have generic drivers rated for 4.2 - 4.5 V maximum.
Surefire (high-end, not spoken about much in here) lights are mostly designed for primary cells and some of the Solarforce replacement dropins too. This is somewhat due to Surefire's history as a military supplier. Primary cells are often favoured by the military and many outdoors types who don't want to bother with rechargeables when on the go. Many of these and other boutique light run 6 - 8.4 V capable drivers.
But unless you can find published specs for your light, you are risking frying the driver circuit.
Just adding to what L4M4 told you, you can usually use an 18650 cell in a light designed for 2 CR123 primaries, but sometimes the tube is too narrow for the larger cell to fit.
There are a range of options regarding "boring" out Surefire lights but that's probably something you'll find more information about on CPF where they tend to talk a lot more about the high-end stuff.
Oh, and welcome to BLF!
The driver is the circuit board that goes in between the batteries and the LED emitter. A quick google search for "flashlight driver" should net you plenty of results.
A driver designed for two CR123 cells won't necessarily be dimmer on 1 18650 as they still are able to deliver a voltage suitable for driving the LED. The LED may require a voltage of about 3.5V or less depending on how hard it is being driven but the actual number will vary with the type of emitter. There is a small voltage drop due to the requirements of the driver.
When your battery voltage drops and the driver can no longer provide a high enough voltage the light will start to get dimmer. By this stage you can usually tell that your battery is getting flat because the light is not as bright as it used to be.
hope this helps.
if the 2xcr123 light runs a standard led and the driver is a buck then yes.that is if the 18650 fits.might need a 17670.
wont hurt to try.
Agreed with you! Because to put that into perspective, to get the same voltage as one EBL 18650 battery, you'd need the equivalent of 2.5 AA batteries attached end to end
I mean, that is indeed correct, but this is a thread about CR123s 
Ah ok ok.
Anyway, just make sure that you do not necro old threads too much 
some lights [i have a blue laser] are designed for either one
otherwise no
i mean 18650 will not fit a 1x or 2x cr123a space
2 cr123as have 6.4V